In an iron bath-type melting furnace, carbon in hot metal and/or a carbonaceous material supplied is combusted by oxygen injection to melt a raw material iron source and to thereby manufacture molten iron. The molten iron in the furnace needs to be discharged outside the furnace. Various methods for discharging the molten iron have been proposed but all have had problems such as those described below. Currently, there is no established method for discharging the molten iron.
Many proposals have been made on methods that use conventional converter-type furnaces as the molten bath-type melting furnaces (e.g., refer to PTL 1). However, when a converter-type furnace is used as an iron bath-type melting furnace, oxygen injection is stopped (in other words, production of the molten iron is interrupted) and the furnace body is tilted to discharge molten iron and molten slag (hereinafter also simply referred to as “slag”). Thus, the productivity of the molten iron decreases due to this discontinuation of injection. Moreover, since the temperature of molten iron in the furnace decreases due to the heat loss from the furnace body surface to the ambient air during tapping, operation for compensating the temperature decrease and raising the temperature must be conducted for next injection before raw material iron sources are charged. This has resulted in a further decrease in productivity of molten iron.
Another example of an iron bath-type melting furnace which has been disclosed is a continuous tapping-type melting furnace in which tap holes are formed in a side wall of the furnace bottom, a refractory structure called a forehearth is provided in the front of the tap holes, and a channel for continuous tapping extending from the tap holes to the tap position leading to a tapping runner is provided inside the refractory structure (forehearth) (refer to PTL 2). However, according to the continuous tapping-type melting furnace, heat loss between the forehearth and the tapping runner is large, requiring heating with an auxiliary burner or the like. Moreover, when melting and injection is discontinued due to facility trouble in, for example, raw material supply facilities or oxygen supply facilities, hot metal and molten slag will solidify and clog between the forehearth and the tapping runner, requiring many hours and high cost for recovery. Furthermore, since the hot metal is discharged continuously instead of in batches, it takes time for a ladle to receive the hot metal in an amount needed for use in the subsequent steel-making step which is a batch process. Thus cooling of the hot metal discharged early cannot be ignored and, at worst, the hot metal may become solidified in the ladle.
Also disclosed is an iron bath-type melting reducing furnace according to which hot metal and molten slag are intermittently discharged outside the furnace through a fixed tap hole and a fixed slag-off hole in a furnace wall while keeping the furnace body erect (refer to PTL 3). However, it is anticipated that when hot metal and molten slag are discharged through the fixed tap hole and the slag-off hole, block refractories of the tap hole (hereinafter also simply referred to as “refractories”) and the refractories of the slag-off hole will be significantly worn due to the hot metal flow and oxidation caused by high-FeO-containing slag, respectively, thereby leading to a problem of an extremely short furnace life. In order to extend the furnace life, the refractories of the tap hole and the slag-off hole may be repaired, but this requires many hours of shut down and results in a significant decrease in productivity.